That doesn't sound right to me.
There may be something wrong with the receiver as the problem only occurs when it warms up.
On a different note though you'd be better connecting the PS3 via HDMI and doing away with the optical lead.
HDMI sends both audio and video so you've no need for the optical connection.
The only reason you need the optical cable for the SkyHD box is that Sky have disabled audio over HDMI. There's speculation that they'll re-enable that in the future firmware update though.